I’ve been playing around with the code, and arrived at this:
In line 148 of lib/email/message_builder.rb, this would result in what I would like to do:
      if allow_reply_by_email?
        result['X-Discourse-Reply-Key'] = reply_key
        result['Reply-To'] = "test1@test.com"
        result['CC'] = reply_by_email_address
      else
        result['Reply-To'] = from_value
      end
However, now I’m trying to figure out what the ‘sender email address’ variable would be in ruby world…